The invention disclosed herein pertains generally to telecommunications switching systems and more particularly to a system having the capability of simultaneously switching voice information and data information coupled to such system via separate subscriber voice lines and data lines.
The switching system described herein is a PCM time division multiplex system designed to accept analog voice information and convert it into corresponding digital representations for subsequent network switching and reconversion into the analog form. Such a PCM switching technology is explained in "A review of Telecommunications Switching Concepts--Part 1 and Part 2 thereof, published in Telecommunications February, 1976 and March, 1976 respectively.
Such systems allocate dedicated time paths for carrying digital words representative of corresponding analog voice samples. The majority of these systems are designed for switching analog voice information in the range of about 300 Hz to 4 KHz. To obtain an adequate sampling rate, such a PCM system samples a plurality of analog voice signals at a typical 8 KHz rate, i.e. every 125 microseconds. The samples are encoded into eight-bit digital words.
Early attempts at switching digital data information exclusively through the voice channels of such PCM systems were successful, but only at the expense of denying the use of those channels for PCM voice transmission. Without equipment to synchronize the transmitted digital data with the switching system sampling rate, only asynchronous data up to 4 kilobits per second (Kbs) could be sampled and switched accordingly.
While technical developments have led to the availability of synchronizing equipment capable of being interfaced with the voice ports for collecting and storing the data bits and supplying the same to the PCM system in synchronism with the system clock, such advancements have allowed the systems to switch data rates only in the order of about 8 Kbs or less, and with a sacrifice in voice channel capacity.
One of the most popular methods o transmitting digital data is by way of the data modem interface. Such a device adapts a remote data terminal to the voice grade telephone line of a switching system by remotely transforming the digital data into frequency modulated analog signals, all within the voice frequency spectrum, and transmitting the same over the telephone lines. In this application the analog voice port is utilized, without modification, to couple the frequency modulated signals to the switching system for processing and switching as any other voice signal. In establishing a switching system network connection the data terminal user dials a destination directory number, and after a successful cut-through to the destination equipment, data information may be transferred but again to the exclusion of voice information transmission. The intermediate switching system is unaware whether voice or frequency modulated data signals are being switched.
Other methods of switching digital information involve "stealing" a few voice PCM channel time slots and inserting therein digital data bits. While this method allows one to switch voice and data information simultaneously, the quality of the voice transmission is compromised. This compromise in quality is obvious in view that there are fewer digital bits representing quantum changes in the corresponding analog sample.
It is thus seen that attempts have been made to modify present PCM digital switching systems to accommodate the transmission of digital data. However, these modifications, by and large, involve a compromise in the transmission quality or the number of voice transmission channels.
It is, therefore, a primary object of the present invention to provide a PCM switching system having the capability of switching voice and data information simultaneously without a compromise in voice quality or the number of voice channels.
It is another object of the invention to provide a PCM telecommunications switching system capable of switching asynchronous data appearing at the data ports, at rates in excess of 8 Kbs.
It is another object of the present invention to provide a PCM telecommunication switching system with data port time channels uniquely associated with voice port time channels so that a network connection established for a voice connection automatically establishes data connection to the same destination. This permits communication with a distant location simultaneously using a telephone set for voice and a data terminal for data.
A feature of the present invention is to integrate voice and data hardware so that the data switching capabilities of the system may be invoked without the use of special services or access codes.
The foregoing as well as other objects and advantages will become apparent by referring to the detailed description of the preferred embodiment, which follows hereinafter, together with the appended drawings.